题目

_______ you can sleep well, you will lose the ability to focus, plan and stay motivated after one or two nights. A . Once B . Unless C . If D . When 答案：B把一条盘在地上的长为A的匀质铁链向上刚好拉直时，它的重心升高了________；如图所示，一个边长为A的匀质立方体，绕bc棱翻到使对角面AbcD处于竖直面时，其重心位置升高了________．